Winehouse, who won five Grammy awards for her breakthrough album, Back to Black, died in July 2011 at the age of 27.

An inquest recorded a verdict of misadventure after finding that she had 416mg of alcohol per decilitre in her blood – more than five times the legal drink-drive limit and enough to make her comatose and depress her respiratory system.

According to her brother, who was speaking to mark the opening of an exhibition dedicated to his sister's life at the Jewish Museum in Camden, north London, her system had been fatally weakened by years of bulimia, a disease in which bouts of extreme overeating are followed by depression and self-induced vomiting. "Had she not have had an eating disorder, she would have been physically stronger," he said.

Alex Winehouse revealed that Amy had developed bulimia in her late teens and had never shaken off the illness. He explained that, as a 17-year-old, his sister had a group of friends who "were all doing it. They'd put loads of rich sauces on their food, scarf it down and throw it up. They stopped doing it, but Amy never really stopped. We all knew she was doing it but it's almost impossible [to tackle], especially if you're not talking about it."

According to Beat, the world's largest eating disorders charity, there is a lack of data detailing how many people in the UK suffer from an eating disorder. Although the Department of Health provides hospital episode statistics, these include only those affected by eating disorders who are being treated as NHS inpatients. As a result, the figures omit all those who have not come forward, have not been diagnosed, are receiving private treatment or are being treated as an outpatient or in the community.

It is estimated that, of those with eating disorders, 40% are bulimic. Bulimia is associated with severe medical complications. Anorexia has the highest mortality rate of any psychiatric disorder. Research has found that 20% of anorexia sufferers will die prematurely.

After Winehouse's death, her family set up a foundation in her name to curb misuse of drug and alcohol by young people. The Amy Winehouse Foundation is run by Alex and his father, Mitch, a singer and former black cab driver.

The charity recently donated money to Beat to enable it to continue running an internet forum with a dedicated moderator.

Alex Winehouse said: "We had to support eating disorder charities because no one talks about it. The situation in this country is that about five or six years ago there were around 10-15 eating disorder charities out there. Now there's only three, one of which is exclusively for young men.

"Beat was in real need of an online forum … so that there's always someone there to talk to. I just want to try to raise awareness of bulimia. It's a real dark, dark issue."

As an exhibition opens about her family life, Amy Winehouse's brother Alex talks in his first major interview about the girl who became a superstar – and reveals what he thinks really killed her. Elizabeth Day reports